Embodiments of our invention relate to hetero phase polymer compositions having three main polymeric components comprising a polypropylene component, a modifier component and a compatibilizer component. The compatibilizer component typically imparts a greater degree of compatibility between the polypropylene component and modifier component phases, yielding improved physical properties, notably impact strength. Non-polymeric components may also be present in the compositions.
The term hetero phase polymer compositions refers to the presence of at least two phases: a continuous or matrix phase and a discontinuous or dispersed phase distributed within the matrix phase.
Hetero phase polymer compositions include compositions generally referred to as Thermoplastic Olefin (TPO) Compositions, which are blends of polypropylene, modifier and optionally, non-polymeric components such as fillers and other compounding ingredients. TPOs are multiphase polymer blends where the polypropylene forms a continuous matrix phase and the modifier component, generally derived from an ethylene containing interpolymer, is the dispersed phase. The polypropylene matrix imparts tensile strength and chemical resistance to the TPO, while the ethylene polymer imparts flexibility and impact resistance. Generally speaking, TPOs have a dispersed phase which is not, or only modestly, cross-linked.
Hetero phase polymer compositions also include compositions generally referred to as thermoplastic vulcanizates (TPVs), which are also blends of polypropylene, modifier and optionally non-polymeric components such as fillers and other compounding ingredients, with the exception that thermoplastic vulcanizates are multiphase polymer blends where the dispersed modifier component is cross-linked or “vulcanized” to provide a rubber-like resilience to the composition. As a result of the increased coherence of the dispersed, crosslinked modifier, its share of the total composition may be increased to levels higher than for TPO's.
Traditionally, highly amorphous, very low density ethylene-propylene copolymers (EP) and ethylene-propylene-diene terpolymers (EPDM) have been used as the modifier component in hetero phase compositions. These EPs or EPDMs generally have a high viscosity expressed in Mooney units. The major market for TPOs is in the manufacture of automotive parts, especially bumper fascia. Other applications include automotive interior components such as door skin, air bag cover, side pillars and the like. These parts are generally made using an injection molding processes. To increase efficiency and reduce costs it is necessary to decrease molding times and reduce wall thickness in the molds. To accomplish these goals, manufacturers have turned to high melt flow polypropylenes (Melt Flow Rate>35 g/10 min.). These high melt flow rate (MFR) resins are low in molecular weight and consequently difficult to toughen, resulting in aforementioned products that have low impact strength.